1. Field
Aspects of the present invention relate to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to an electrolyte for a lithium secondary battery capable of improving battery life and high-temperature storage characteristics, as well as a lithium secondary battery including the same.
2. Description of the Related Art
Lithium-ion batteries (LIBs) have been developed and used with a focus on small electronic devices and portable information technology (IT) devices because of their high energy density per unit weight and ease of design. Recently, according to developments in electric vehicle power sources and alternative energies, medium and large sized lithium ion batteries are highly anticipated to also be used as power storage sources capable of storing produced electricity.
A lithium secondary battery is composed of a cathode, an anode, an electrolyte, and a separator. During discharging, an oxidation reaction occurs at the anode due to extraction of lithium ions and a reduction reaction occurs at the cathode due to insertion of lithium ions. During charging, an oxidation reaction occurs at the cathode due to extraction of lithium ions and a reduction reaction occurs at the anode due to insertion of lithium ions. The electrolyte does not exhibit electron conductivity and only exhibits ion conductivity, and functions to transfer lithium ions between the cathode and the anode.
Lithium ions inserted into an electrode in the battery may achieve charge neutrality with electrons moved into the electrode and thus are a medium for storing electric energy in the electrode. Therefore, the amount of ions inserted into an electrode for achieving charge neutrality determines the amount of electric energy storable in the battery. Although basic performance characteristics, such as operating voltage and energy density of the lithium secondary battery, are determined by materials constituting the cathode and the anode, it is necessary for the electrolyte to have high ionic conductivity, electrochemical stability, and thermal stability in order to obtain excellent battery performances.
A lithium salt and an organic solvent are used as components of the electrolyte. In consideration of the reduction reaction with the anode and the oxidation reaction with the cathode, the electrolyte has to be electrochemically stable in each corresponding electric potential region.
Meanwhile, electrode active materials for high voltages are being used as the field of lithium secondary batteries is expanded to the fields of electric vehicles and electric power storage. Since an anode active material with low electric potential and a cathode active material with high electric potential are used for the cathode and anode electrodes, the potential window of the electrolyte is narrower than those of the active materials. Thus, the electrolyte is exposed to an environment in which the electrolyte is easily decomposed on surfaces of the cathode and anode electrodes. Also, the lithium secondary battery, when used in an electric vehicle or an electric power storage device, is easily externally exposed to a high-temperature environment, and the temperature of the battery may increase due to instantaneous charge and discharge. Therefore, life of the battery is reduced and the amount of storable energy may be decreased in the high-temperature environment.
Research has been conducted to prevent oxidation of an electrolyte caused by an electrode active material, by forming a thin protective layer on an electrode surface, as one of various research areas related to finding a battery material capable of exhibiting high energy storage and long life in high voltage and high-temperature environments.
With a graphite anode it has been known that a solid electrolyte interphase is formed on the surface of an anode active material during initial charging when an appropriate electrolyte, or an appropriate electrolyte additive, is used, and the solid electrolyte interphase prevents direct contact between the electrolyte and the anode active material to prevent decomposition of the electrolyte. On the other hand, with respect to a cathode, an electrolyte additive is known as an overcharge inhibitor that stops battery operation by forming on a surface of the cathode a thick solid electrolyte interphase to prevent penetration of lithium ions when the cell voltage is increased above a specified voltage.
A cathode solid electrolyte interphase is becoming increasingly necessary as cathode active materials are operated at increasingly high voltages. Research results have reported that when the concentration of an additive used as an overcharge inhibitor is greatly reduced, a thin solid electrolyte interphase is formed on a cathode surface, thus enabling improvement of battery life. However, since the solid electrolyte interphase is nonpolar, penetration of lithium ions is not easy. Therefore, the solid electrolyte interphase will adversely affect battery characteristics.